The phenomenon of compressor stall has become an important limiting factor in the operation of gas turbine engines as their performance characteristics have improved. In modern gas turbine engines, upon acceleration or under high altitude and lower speed flight conditions, unstable flow may develop in the compressor which can lead to a stall with a resultant increase in turbine temperature and mechanical vibration along with a simultaneous reduction in cooling air supplied to the turbine. These conditions describe "compressor stall" and can lead to turbine failure if the compressor stall is not recognized and corrective action not taken. Turbine failure during engine operation can lead to severe engine and aircraft damage.
It is known that during transient engine operation, such as during engine acceleration or deceleration, the thermal characteristics (the temperatures of the gas flow and metals) of the compressor components are not the same as they are during steady-state conditions. During an engine transient condition, there is a heat transfer between the gas flowing through the compressor and the metallic case, blades and stators of the compressor. Heat flows from the metallic case, blades and stators to the gas flowing through the compressor, thus increasing the gas temperature. This results in the temperature of the gas flowing through compressor being higher at the exit end of the compressor than in the front of the compressor for a given speed than the corresponding steady-state temperature. As a result, there is a difference or mismatch in the pressure ratios and thermal characteristics between the front stages of the compressor and the aft stages of the compressor due to heat transfer effects, which adversely affects compressor stability. It is undesirable to have a mismatch of gas flow thermal and pressure characteristics through the compressor stages as it may lead to the aft stages operating at lower corrected speeds as the speed is correlated with the gas flow temperature. The compressor's stability is compromised during the mismatch of gas flow characteristics as the front stages may have a much higher pressure ratio than the aft stages leading to a potential stall condition. For example, during deceleration from high to low power, heat transfer effects from the hot metal parts (case and airfoils) reduces the rate with which the gas flow temperature decreases from the temperature corresponding to the steady-state characteristic. Without heat transfer effects, the gas flow temperature would decrease at a faster rate and reach a lower temperature. The heat transfer effect tends to lower the corrected compressor speed of the rear or aft stages of the compressor. This detracts from the suction capability of the aft stages. In other words, the gas flow experiences a resistance as it flows axially downstream the engine flow path. The forward stages thus get back pressurized and, in turn, the operating line for those forward stages approaches the stall line, decreasing the stall margin.
The thermal characteristics of the engine can be synthesized or calculated using sensed parameters to control the deflection of the stator blades and fuel flow to provide an acceptable level of stall margin during acceleration and subsequent thermal nonequilibrium condition. For example, U.S. Pat. No. 5,165,845, assigned to the assignee of the present invention, discloses a control system for modifying engine airflow geometry to increase the compressor stall margin during engine acceleration by synthesizing the thermal enlargement of critical compressor stages. This thermal enlargement provides a measure of the temporary increase in blade-case clearance during acceleration. The change in clearance is used to provide a signal, which in turn increases stator vane deflection during acceleration until the clearance returns to a nominal level.
The above-described prior art method increases stall margin during high power thermal stabilization when tip clearances get large. However, even though prior art methods compensate for stall margin loss due to increased tip clearances, stall margin loss due to mismatch of gas flow characteristics aggravated by heat transfer effects have not been adequately understood and addressed. Other prior art control systems to increase stall margin, due to their reliance on a plurality of flow measurements, are complex to implement and are not robust.